Igneous Rocks and Minerals

1. Igneous Rocks and Minerals

2. Magma and Igneous Rocks Magma Complex liquids � solutions of Si, Al, O etc Temperatures up to 1600 oC Originate by melting of the earth�s mantle and crust Have lower density than the solids from which they are derived therefore they rise

3. Cooling of Magma When magma cools slowly crystals form Within the earth � INTRUSIVE Crystals fairly large � visible to naked eye Various types of intrusive rocks Dikes, sills, loppoliths, laccoliths, stocks, batholiths

4. Cooling of Magma When magma reaches the earth�s surface called LAVA Forms extrusive rocks (AKA volcanic) Lava flows, volcanic edifices, pyroclastic rocks

5. Grain Size and Cooling Rate Coarse grained igneous rocks Generally cooled slowly Fine-grained igneous rocks Generally interpreted to have cooled quickly Glassy igneous rocks (obsidian) Cooled so quickly that crystals could not form

6. Cooling Rate and Grain Size Some rocks have coarse grains in a finer grained matrix Coarse grains = phenocrysts Fine grains = groundmass Called PORPHYRITIC texture

7. Mafic, Intermediate and Felsic Source of magma affects composition Magmas derived from the mantle generally poor in Si, Al and rich in Mg and Fe � MAFIC magmas Magmas derived from the crust rich in Si and Al and poor in Fe and Mg - FELSIC

8. Other Classifications Petrologists also use the amount of Na2O and K2O Especially to classify volcanic rocks

9. Melting Temperature of Magma Ultramafic magmas Komatiite � very high MP (1600 oC), low viscosity (close to water) Mafic / basic magmas Basalt � MP 1200 � 1400 oC, low viscosity (similar to warm honey)

10. Melting Temperature of Magma Intermediate magmas Andesite, MP 900 � 1200 oC, intermediate viscosity (cold honey) Felsic magmas Rhyolite / granite, MP ~ 670 � 900 oC, high viscosity (similar to tar)

11. Viscosity, MP and Composition Magmas are complex solutions Mainly Si, Al, O with lesser Fe, Mg, etc Si, Al and O make chains � polymers The more polymerized a melt the longer the chains Polymerized melts have high viscosity Polymerized melts are Si- and Al-rich Polymerized melts have low melting points

12. Volatiles in Magma Under pressure magma can hold dissolved volatiles (H2O, CO2 etc) As pressure decreases these exsolve Form bubbles Same way that beer bubbles when you open the bottle! Magma = beer? Degassing of lava makes vesicles

13. Rock forming minerals Silica minerals Composition SiO2 Various polymorphs (P/T regime) alpha quartz - trigonal (low T) beta quartz - hexagonal (high T) tridymite cristobalite Beta quartz is the form that actually crystallises but on cooling it transforms to alpha quartz goes through a phase transition

14. Feldspars Most abundant mineral group Framework aluminosilicates tetrahedra of Si and Al to balance charge incorporate Na, Ca and K also small amounts of Ba, Sr, Rb KAlSi3O8 - Orthoclase also microcline and sanidine polymorphs NaAlSi3O8 - albite CaAl2Si2O8 - Anorthite

15. Feldspars - 2 Rare in pure form occur as solid solutions end members mix mixing controlled by match of structure and cation sizes Plagioclase series solid solution between An and Ab An0-10 - albite An10-30 - oligoclase An30-50 - andesine An50-70 - labradorite An70-90 - bytownite An90-100 - anorthite

16. Feldspars - 3 Alkali feldspar series Mixing between albite and orthoclase High temperature Mixing between orthoclase and anorthite limited Why? Ionic radii Ca2+ - 1 angstrom Na+ - 1 angstrom K+ - 1.3 angstrom Because of solid solution feldspars show zoning and exsolution

17. Pyroxenes Most abundant ferromagnesian mineral (Fe- and Mg-rich) Most contain Fe2+ and Mg Exceptions Jadeite - NaAlSi2O6 Aegerine - NaFe3+Si2O6 Single chain silicates endless chains of SiO4 tetrahedra chains held together by interstitial cations mainly Ca2+ Mg2+ and Fe2+ lesser Al3+, Ti4+, Mn2+ Cr3+, Na+

18. Pyroxenes - 2 Ca-rich pyroxenes Monoclinic symmetry CLINOPYROXENES Ca-poor pyroxenes Orthorhombic symmetry ORTHOPYROXENES Exception Pigeonite Has monoclinic symmetry Relatively poor in ca Found in some basalts

19. Pyroxenes - 2 Solid solutions Complete Between orthorhombic end members Between monoclinic end members Incomplete between ortho and mono

20. Pyroxenes - 3 End members names Calcic (monoclinic) pyroxenes Diopside CaMgSi2O6 Hedenbergite CaFeSi2O6 Tschermaks molecule CaAl2SiO6 Low Ca monoclinic pyroxenes Pigeonite (Mg, Fe, Ca) Si2O6 Low Ca (orthorhombic) pxroxenes Enstatite Mg2Si2O6 Ferrosalite Fe2Si2O6 Sodic (monoclinic) pyroxenes Jadeite NaAlSi2O6 Aegerine NaFe3+Si2O6

21. Pyroxenes - 4 Most common solid solution Between diopside and hedenbergite Natural pyroxenes have other cations Al, Ti, Fe3+, Na Solid solution between enstatite and ferrosalite Orthopyroxene series Minerals in these two series coexist Compositions depend on temperature Geothermometer Aegerine forms a solid solution with augite Presence of pyroxene - magma was poor in water

22. Olivine Common in Si-poor rocks basalt family End members Forsterite Mg2SiO4 Fayalite - Fe2SiO4 show complete solid solution compositions quoted in % Fo Mg olivine incompatible with quartz Fe olivine occurs with quartz

23. Feldspathoids Characteristic of very Si-poor magmas Similar in composition to feldspars but deficient in silica Most common feldspathoids Nepheline AnAlSiO4 Leucite KAlSi2O6 Sodalite Na3Al3Si3O12.NaCl

24. Amphibole Group VERY COMPLEX Large number of end members Basic composition AX2Y5Si8O22(OH)2 A = vacant or Na or K X = Ca, Na, Fe + Mg Y= Fe2+, Fe3+, Mg or Al Al substitutes for Si in the tetrahedral site There are many possible substitutions

25. Amphibole Group - 2 Calcic amphibole Tremolite Ferroactinolite Tschermakite Edenite Low Ca clinoamphiboles Cummingtonite Grunerite

26. Amphibole Group - 3 Simplest amphibole series Tremolite - ferroactinolite Quite common in igneous rocks Most common amphibole is the calcic amphibole hornblende Ca-amph will crystallise directly if enough water available Otherwise results from subsolidus reaction of Ca-pyroxene with fluid Riebeckite occurs in some sodic granites

27. Micas Sheet silicates Muscovite - KAl3Si3O10(OH)2 Limited substitution of K by Na and Li Biotite - solid solution of Annite - KFe3AlSi3O10(OH)2 Phlogopite - KMg3AlSi3O10(OH)2

28. Micas - 2 Contain more H2O than amphibole Form in more water rich magma

29. Accessory silicates Garnet (Fe Mg Ca Mn)3Al2Si3O12 Characteristic of Al-rich silicic magmas Occurs with muscovite Tourmaline Hydrous Na, mg, Fe borosilicate Very complex Common as a late mineral in granite Zircon - ZrSiO4 Often contains U Important in radiometric dating

30. Accessory silicates - 2 Titanite (aka sphene) CaTiSiO5 Common in mafic rocks Epidote Ca2Al2Fe3+Si3O12 (OH) occurs as a primary mineral in some high pressure granites useful as a geobarometer more commonly as an alteration product of feldspar or cpx

31. Oxides Fe - Ti oxides Magnetite Fe3O4 Ilmenite - hematite solid solution Ilmenite - FeTiO3 Hematite - Fe2O3 Composition of coexisting hematite and ilmenite - magnetite (ss) very sensitive to oxygen content in the magma Spinel - complex ss with Fe2+, Fe3+, Mg, Al, Cr Common in mafic - ultramafic rocks

32. Sulphides Generally solid solutions of Fe - Ni- Cu and S Pentlandite Pyrrhotite Pyrite Chalcopyrite Monosulphide solid solution

33. Phosphates Apatite Ca5(PO4)3(OH,F,Cl) Can be found in almost every igneous rock Monazite CePO4 Contains abundant REE and radioactive cations Important in raiometric dating Surrounded by pleochroic haloes in when included in amph and mica

34. Classification IUGS classification Based on the proportions of felsic minerals Quartz Plagioclase Alkali feldspar Feldspathoids

35. Plutonic rock classification

36. Volcanic QAPF diagram When modal data is available this classification must be used it is equivalent to the plutonic classification

37. Other Helpful Classification Tips Granite Alkali feldspar common Quartz Na-rich plagioclase Amphibole Biotite Sometimes muscovite Diorite / Andesite Alkali feldspar minor to rare Quartz (not always present) Intermediate plag Amphibole Minor biotite Muscovite very rare

38. Other Helpful Classification Tips Basalt / Gabbro Calcic plagioclase Clinopyroxene Orthopyroxene Olivine Amphibole Rare biotite Komatiite / peridotite Olivine Orthopyroxene Clinopyroxene

39. Order of Crystallization